It has since long been acknowledged that seeds derived through cross-pollination between different varieties of one species, give rise to offspring with better characteristics in terms of yield, environmental fitness, and disease resistance, when compared with the offspring of seeds derived through self-pollination. This effect is generally referred to as the heterosis effect. For this reason, it is an object for the seed industry to obtain hybrid seed in as many agricultural and horticultural crops as possible, because of their higher commercial value.
Unfortunately, however, many crop plants bear the male and female reproductive organs on the same individual, which strongly promotes self-pollination over cross-pollination. Therefore, in order to obtain seeds derived from cross-pollination, it would be desirable to have acceptor plants which are unable to self-pollinate, due to the absence of (properly functioning) pollen. These male-sterile plants or female parents, are then used in cross-fertilization with a male-fertile donor plant to produce hybrid seed.
For the production of hybrid seed on a large scale usually the male-sterile plants and the male-fertile plants are grown together in the field and allowed to cross-pollinate, whereafter the hybrid seed is selected. Depending on the type of male-sterile plants that are used, the selection or separation of hybrid seed is performed before harvesting, i.e. by destroying or removing the male-fertile donor plants which produce non-hybrid seeds, or after harvesting, e.g. on the basis of a marker, such as seed color in maize, or another easily perceptible phenotype. The pre-harvesting selection is possible when the male-fertile parent can be distinguished from the male-sterile parent plant and can subsequently be removed or destroyed. Alternatively, when the male-sterility locus is closely linked to a selectable marker (such as a herbicide resistance), the male-sterile plants, which carry the hybrid seeds, can outcompete the male-fertile plants by applying the appropriate selective pressure.
As male-sterile parental line, use is made of e.g. physically emasculated plants, or if available natural cytoplasmic or nuclear encoded male-sterile mutants. Such naturally male-sterile plants have their disadvantages, be it the very laborious preparation, the presence of additional undesired characteristics, the difficulty of maintenance and propagation, the unpredictable inheritance, or the limited availability of natural male-sterile mutants in commercially interesting crops.
Only recently, genetically engineered nuclear encoded male-sterile plants are known that can be used for the production of hybrid seeds and which lack at least some of the disadvantages of most natural male-sterile mutants.